1. Digital Transmission Outlines:- Multiplexing FDM TDM WDM
Chapter review
Chapter revision
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

2. Introduction Or Or Definition
1. The transmission of information from one or
more source to one or more destination over
the same medium. Or
2. The process which two or more signals
are combined for transmission over a single
communications path. Or
3. A process where multiple analog message
signals or digital data streams are combined into
one signal with the aim is to share an
expensive resource.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

3. What is multiplexing and demultiplexing?
Multiplexing – the process of combining together many separate signals to send them over the same transmission media.
Demultiplexing – reverses the multiplexing at the receiver
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

4. Cont’d...
Multiplexing allows several analog signals to be processed by one analog-to-digital converter (ADC)
E.g. Several phone calls may be transferred using one wire.
The multiplexed signal is transmitted over a communication channel which may be a physical transmission medium.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

5. Cont’d...
The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred.
A reverse process, known as demultiplexing, can extract the original channels on the receiver side.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

6. Cont’d...
A multiplexing technique may be further extended into a multiple access method or channel access method.
E.g. Time Division Multiple Access (TDMA)
Multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

7. Cont’d... Concept of multiplexing EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

8. Why needs multiplexing?
Make the communications very economical by transmitting thousands of independent signals over a single transmission line.
Three predominant ways to multiplex
Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
Wavelength Division Multiplexing (WDM)
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

9. Frequency Division Multiplexing (FDM)
Predominantly used in analog communications.
Widely used in FM stereo broadcast.
Compatibility with mono-receivers.
Requires only a slight increase in BW.
Great advantage in increasing system capacity.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

10. Cont’d... Transmitting end of an FDM System
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

11. Cont’d... Process Each signal assigned s different carrier frequency.
Modulated carrier frequencies are combined for transmission over a single line by MUX.
At Rx, a DEMUX separates channels by their frequencies & routes them to the proper end users.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

12. Time Division Multiplexing (TDM)
Offers greater system improvement.
Capable of combining various protocols & different types of signals onto a single high speed transmission link.
E.g. voice and data
More efficient than FDM
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

13. Cont’d... Process The transmission must be digital in nature.
To transmit telephone conversations (voice is analog signal)
Analog signal is converted to digital and transmitted
Reconverted into analog at the Rx telephone.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

14. Cont’d... Drawbacks Greater complexity.
Greater transmission BW required.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

15. Wavelength Division Multiplexing (WDM)
Cost effective way to increase the capacity of fiber optic communications.
Make use of the optical fiber’s available BW by multiplexing many wavelengths along a single mode optical fiber channel.
Each wavelength of light can transmit encoded information at the optimum data rate.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

16. REVISION 1) Define M-ary Encoding
2) Find the number of voltage levels which can represent an analog signal with
8 bit per sample
12 bit per sample
3) Calculate the rate of serial data flow, given the one bit time of 2μs.
4) A modulator transmits symbols, each of which has 64 different possible states, 10K times per second. Determine the bit rate.

17. REVISION (Cont’d)
5) If an audio frequency signal is band limited to a range of 300 Hz to 3.4 kHz, find
a) The minimum frequency required to sample the audio signal in order to be transmitted without aliasing.
b) The Nyquist interval
6) Digital modulation can be grouped as Amplitude-shift keying (ASK), Frequency-shift keying (FSK) and Phase-shift keying (PSK). Briefly explain each of them with your own words
7) What is a constellation diagram?

18. REVISION (Cont’d) Constellation diagram
Graphical representation of the complex envelope of each possible symbol state.
The x-axis represents the in-phase component and the y-axis represents the quadrature component of the complex envelope.
The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how easily a receiver can differentiate between them.

19. REVISION (Cont’d)
8) A typical dial-up telephone connection has a bandwidth of 3 kHz and a signal to noise ratio of 30 dB. Calculate the Shannon limit.
9) For the digital message , sketch the waveform for the following:
a. ASK
b. FSK
c. PSK
d. QAM
10) For 16-PSK system, operating with an information bit rate of 32 kbps, determine:
a. Baud
b. Minimum bandwidth
c. Bandwidth efficiency

20. REVISION (Cont’d)
11) List down the factors in choosing the digital modulation technique for a specific application.
a) Error performance
b) Bandwidth efficiency
c) Implementation complexity and cost
d) Effects of fading, interference and channel
e) Transmitted power
12) What is the sampling process?
A process of taking samples of information signal at a rate of Nyquist’s sampling frequency.
The original information signal can be reconstructed at the receiver with minimal distortion if the sampling rate follows the Nyquist.

21. REVISION (Cont’d)
13)Differentiate between Natural Sampling & Flat top sampling.
14)Sketch the waveform of pulse modulation for
Pulse Amplitude Modulation (PAM)
Pulse Width Modulation (PWM)
Pulse Position Modulation (PPM)
15) A CD audio laser disk has a frequency bandwidth of 20 Hz to 20 kHz. What is the minimum sample rate required to satisfy the Nyquist sampling rate?

22. REVISION (Cont’d) 16) What is meant by quantization?
17) Sketch the diagram that shown the quantization error.
18) Explain the companding process.
19) A PCM system requires 72 dB of dynamic range. The input frequency is 10 kHz.
(a) Determine the number of sample bits required.
(b) Specify the minimum sample frequency to satisfy the Nyquist sampling frequency.